One model of an automatic implantable defibrillator is being tested clinically, and progress shows promise for future application to patients at high risk for sudden death syndrome due to ventricular fibrillation. However, a major problem exists, namely the detection systems used for both experimental animal and human use misdiagnoses rhythm disorders. We propose herein to develop a system which will have a higher reliability for diagnosis and thereby make the automatic implantable defibrillator safer, more effective, and reliable. This system will also specifically decrease the risk of causing ventricular fibrillation by shocking a ventricular tachycardia or ventricular flutter that are producing a cardiac output. We will use a new impedance technique to detect arterial pulses in the systemic vascular system, which will be superior to presently used or proposed systems, all of which monitor only right ventricular function, usually the ECG. We will implant electrodes on the outer wall of a systemic artery in dogs and measure the pulsatile impedance change. This impedance signal will then be used to verify a preliminary ECG diagnosis of fibrillation. After refinement of electrode design and circuitry, we will implant extra-arterial sensing electrodes in dogs and monitor performance regularly for a 1-year period. Studies will also be made to document complications or undesirable clinical effects from the electrodes. The study will provide equipment and experience for 1) measuring arterial pulse impedance using implanted extra-arterial electrodes, 2) detecting fibrillation by this method, 3) refining the technology, and 4) studying the performance of the implanted electrodes for one year. Successful completion of the project will provide a highly reliable, and hence safer, detection sensor for use in the automatic implantable defibrillator. It will also provide the fundamental data for sensing arterial impedance, which is applicable to other uses, such as blood pressure measurement via pulse-wave velocity.